1. Field of the Invention
This invention relates to fluidized bed heating of materials, including combustion of carbonaceous fuels and/or carbon-based materials such as garbage and heat processing of divided materials.
2. Description of the Prior Art
The concept of Fluidized Bed Combustion (FBC) was established some years ago. In brief, in a fire box, air is blown up through a bed of particulate, non-combustible material such as sand to put the sand in a "fluidized" state in which the sand particles are in constant motion in the air stream, churning and tumbling like a boiling liquid. The particles are heated red hot, to about 900.degree. Fahren-heit, which often is accomplished by injecting a burnable gas into the bed and igniting it. When the bed is hot, the start-up gas can be shut off and the primary fuel fed in.
The heat released by the burning fuel keeps the fluidized bed of sand incandescent, usually at about 1500.degree. Fahrenheit. The hot sand stores heat so well that even cold, wet fuel, such as wet coal, does not appreciably chill the bed. Heat is transferred through direct contact with the hot sand particles, so that water tubes or air ducts immersed in the bed will collect heat much faster than in an ordinary boiler. This means that an FBC unit can be smaller than an ordinary boiler generating the same amount of heat, and can cost less.
FBC boilers burn low-grade fuels, everything from high-sulfur coal to rice hulls. Suitably designed, an FBC system can use wet coal mixed with rock. It can burn peat, heavy oil, oil shale, wood and wood waste including sawdust, urban and industrial trash and garbage, and even sewage sludge.
In addition to the ability to burn low-quality fuels, an FBC can keep emission of sulfur and nitrogen oxides relatively low, eliminating or reducing the need for emission control apparatus. This is in part due to the lower temperature combustion that can take place as compared to a conventional burner. At the lower temperatures, little of the nitrogen in air combines with oxygen to form nitrogen oxide. If the fuel contains sulfur, crushed limestone or dolomite (called "sorbent") is fed into the fluidized bed along with the fuel. The sulfur in the fuel reacts with the calcium in the red hot sorbent to form solid calcium sulfate which is trapped in the bed and removed along with the ash.
As high-quality fuels become scarce and expensive, the ability to use low-quality and low cost fuels becomes increasingly important. Along with the ability to reduce atmospheric pollutants, the FBC is becoming increasingly important as an energy conversion device. The potential of FBC as a device to dispose of solid and liquid waste, as separate from its heat generation role, is largely unexplored, and yet may be highly important, particularly in disposing of hazardous wastes which can be chemically modified by burning or being subjected to relatively high heat. Also largely uninvestigated is the use of FBC burners as process chemical reactors in which the heat of combustion is utilized to promote a chemical or physical change in a substance.
A number of different designs of FBCs have evolved, including variations of firebox designs and of subsystems to facilitate operation. Each design is an attempt to solve certain problems associated with FBCs, or to optimize conditions. Areas of concern are:
1. Feeding of Fuel. PA1 2. Distribution of Fuel. PA1 3. Feeding and Distribution of Limestone Sorbent. PA1 4. Air Distribution, Fluidization and Turbulence. PA1 5. Heat Output. PA1 6. Heat Transfer. PA1 7. Control of Output. PA1 8. Control of Exhaust Gases and Particles. PA1 9. Ash Control. PA1 10. Cost and Complexity.
Feeding of fuel has proved to be one of the most difficult problems encountered. One aspect of the problem arises from the poor reliability of the mechanisms needed to feed coal and other solid fuels. This often results in a trade-off between "sizing" (crushing the fuel to a uniform size) to permit reliable handling, and using complex and inefficient mechanisms intended to handle all sizes of fuel. Another aspect of the same problem is the handling of different kinds and types of fuel with the same mechanism. The prior art generally builds each unit to handle one specific type of fuel, thus limiting the flexibility of use.
Perhaps the most important problem has been the choice of feeding fuel on top of the bed or on the bottom. Feeding on top of the bed is certainly the simplest method--however several drawbacks have been experienced. First, the fuel is being introduced where it can "flash" rather than being brought up to combustion temperature more gradually. The fuel is cold at this point and often wet. Introduction at a hot zone can cool the bed and exhaust gases just where heat transfer is intended. The fuel is also introduced at the point where oxygen is depleted, causing inefficient combustion.
If the fuel contains fine particles, they are very likely to be carried upwards to the heat transfer and exhaust area. The unburned fuel reduces combustion efficiency unless it is returned to the bed. Introduction of the fuel above the bed can also impede heat transfer by masking off the heat transfer tubes.
A more preferable point to introduce fuel is at the bottom of the bed, eliminating many of the problems mentioned above. However, most of the prior art mechanisms to introduce fuel at the bottom of the bed have been extremely complicated and costly, and have been inefficient because of being unable to accomplish even distribution of the fuel into the bed at the bottom thereof.
Uniform distribution of the fuel within the bed is an even greater problem. As the size of FBC units increase, the approach has been to provide multiple fuel feeding points, each with its own costly mechanism. Even multiple points have not fully solved the problem of obtaining evenness of distribution over the entire bed area, and underbed feeding distribution problems have not previously been solved.
Fluidized beds as developed so far tend to be rectangular "boxes" or cylinders, with the bottom perforated for air introduction to the bed. Maintaining even turbulence in a compact box-like bed with side walls and corners is extremely difficult. Equalized supply of air to all "nozzles", and maintenance of full fluidization and even turbulence throughout the bed, is the goal. If not achieved, "slumping" or collapse of the bed can result. Combustion efficiency is highly dependent on even distribution and turbulence.
The importance of maintaining uniform fluidization must be emphasized, particularly when non-uniform fuels, fuels with coarse ash, etc. are burned. If anything dampens the fluidizing action, partial slumping of the bed results, with localized hot spots, or combustion, and corrosion potential resulting. The rectangular box-like present FBCs are difficult configurations in which to maintain this even fluidization and air distribution.
A typical FBC burner has a rectangular or cylindrical firebox, with a relatively high bed volume to grate surface area relationship. As attempts are made to increase the capacity of FBC burners, this relationship limits the capacity for a given size. To increase capacity, grate area must be increased to provide more fuel feeding and air introduction capacity. The fuel must also be spread out evenly, and sufficient air supplied evenly to create uniform turbulence and fluidization.
The rectangular or cylindrical shape is also likely to have a high vertical dimension. Combustion near the top may have insufficient oxygen just when it is needed for maximum efficiency. Introduction of secondary air at that point is complicated and tends to disturb fluidizing action. Increase in residence time of the burning fuel increases the efficiency of combustion, but if the longer residence time is accomplished by a higher bed dimension, the capacity for a given size will not be improved.
One of the most difficult problems in FBC design is to accomodate changes in heat demand or load, as dictated by external conditions. In some way, the FBC heat output must be "turned down" or "turned up" to match the load, and occasionally this must be done quickly. The "turndown ratio" or maximum to minimum capacity is often 5 to 1 or more.
Turndown is accomplished by several means, usually in combination with one another. Often one method alone is not sufficient to cover the range. Fuel feed can be varied to vary output; however the bed temperature must be maintained within a narrow range. Air supply can be varied; however fluidization and turbulence must be kept in a satisfactory range. The height of the bed, or the amount of contact of the bed with water or air heat transfer tubes can be varied; this is an effective control, but difficult to implement.
In each case, the high volume-to-surface area of the fluidized beds in conventional designs causes sluggishness in responding to changing loads. If too much fuel is in the system, turndown is slow. A deep bed mitigates against major changes in air flow, and the height above or immersion of heat transfer tubes in the bed is limited as a control means because of the limited surface area for heat transfer.
Another major problem area of FBCs is the carryover of fine particles of ash, unburned fuel, and limestone in the exhaust gas stream. These fines must be removed before discharge into the atmosphere, to other heat transfer areas, or to direct driven turbines. In addition, the loss of efficiency of having unburned particles of fuel going up the stack is undesirable. Conventionally, particle removal is effected by cyclone separators, filtration, electrostatic precipitation, and the like. Each of these are cumbersome, costly to build, and costly to operate.
Certain aspects of present FBC design make this carryover problem worse: i.e.: top fuel feed, the use of pulverized coal and limestone, and the high air velocity needed to maintain fluidization and complete combustion in relatively deep fluidized beds. It is contemplated that the exhaust gases can be used for directly driving turbines to produce energy. Erosion problems in the turbines due to particles entrained in the exhaust gases are formidable ones and restrict development of direct drive turbines greatly until the particulate problems are solved.
It is believed that the documents listed immediately below contain information which is or might be considered to be material to the examination of this patent application.
______________________________________ Patent No. Inventor Date ______________________________________ 3,578,798 Lappie, et al. 5/18/71 3,863,577 Steever, et al. 2/04/75 4,060,041 Sowards 11/29/77 4,161,917 Jubb 7/24/79 4,177,636 Horgan 12/11/79 4,177,742 Uemura, et al. 12/11/79 4,249,472 Mitchell 2/10/81 4,284,401 Tatebayashi, et al. 8/18/81 4,400,150 Smith, et al. 8/23/83 4,419,330 Ishihara, et al. 12/06/83 ______________________________________
The term "prior art" as used herein or in any statement made by or for applicant(s) means only that any document or thing referred to as prior art bears, directly or inferentially, a date which is earlier than the effective filing date hereof.
No representation is made that any of the above-listed documents is part of the prior art, or that a search has been made, or that no more pertinent information exists.
Copies of the above-listed documents are supplied to the Patent and Trademark Office herewith. Each of the documents relates to Fluidized Bed Combustion devices having one or more of the characteristics set forth above.